(1) Field of the Invention
The present invention relates to a bubble jet type thermal ink-jet technology whereby recording is performed by ejection of ink droplets out of a nozzle by the pressure rise caused by bubbles generated by heat, and in particular relates to an ink-jet recording apparatus for tonal recording.
(2) Description of the Prior Art
For halftone reproduction in the field of ink-jet recording apparatus, there is a method in which the ejected amount of ink droplets is varied. Japanese Utility Model Application Laid-Open Sho 57 No.141043 discloses a circuit which can be applied to varying the amount of ink droplets in a conventional, bubble jet type thermal ink-jet head. This circuit is to vary the ejected amount of ink droplets in conformity with the voltage level of the drive pulse to be applied to the heater. Japanese Patent Application Laid-Open Sho 62 No.261453 discloses an arrangement in which a plurality of heaters are arranged in series in a single pressure chamber and parts of the heaters are selectively turned on at the predetermined timing to heat the ink and generate a bubble of a desired size therein, to thereby eject a desired amount of ink droplets.
When an ink-jet head is configured using the circuit disclosed in Japanese Utility Model Application Laid-Open Sho 57 No.141043, for the case of a single heater, the relationship between the applied energy and the ejected amount of ink droplets as shown in FIG. 8 holds. Actually, there exists a plateau region in which the amount of ink droplets varies very little with increase in applied voltage, in excess of a certain applied voltage level. Therefore, even if the applied voltage is controlled using this circuit, the range in which the amount of ink droplets varies is narrow, hence it is impossible to obtain tonal levels large enough.
According to Japanese Patent Application Laid-Open Sho 62 No.261453, it is possible to change the elected amount of ink droplets over a wide range. However, since independent signals should be applied to drive the multiple heaters, this configuration needs interconnections and driving circuits corresponding to the number of the heaters, hence facing difficulties in making the unit compact and needing more manufacturing cost.
The present invention has been devised to solve the above problems and it is therefore an object of the present invention to provide an ink-jet head which is able to modulate the amount of ink droplets in a wide range and hence provide sufficient tonal representation.
It is another object of the present invention to provide an ink-jet head which keeps ink ejection from being easily broken due to partial disconnection of the interconnections inside the pressure chamber.
In order to achieve the above object, the present invention is configured as follows:
In accordance with the first aspect of the present invention, an ink-jet head for ejecting ink droplets from a nozzle by the pressure caused by bubbles, includes:
a pressure chamber communicating with the nozzle; and
a plurality of heating areas disposed inside the pressure chamber for generating bubbles by heat generation, and is characterized in that heater films arranged in the heating areas are electrically connected in parallel and the surfaces of the heating areas facing the pressure chamber have different thermal efficiencies.
In accordance with the second aspect of the present invention, the ink-jet head having the above first feature is characterized in that each of the heating areas includes an insulating film on the lower side of the heater film and the thermal conductivity of each insulating film is made different from that of the others so as to produce difference in thermal efficiency.
In accordance with the third aspect of the present invention, the ink-jet head having the above first feature is characterized in that each of the heating areas includes an insulating film on the lower side of the heater film and the thickness of each insulating film is made different from that of the others so as to produce difference in thermal efficiency.
In accordance with the fourth aspect of the present invention, the ink-jet head having the above first feature is characterized in that each of the heating areas includes an insulating film on the lower side of the heater film and the ratio of the thermal conductivity to the thickness of the insulating film is made different from that of others so as to produce difference in thermal efficiency.
In accordance with the fifth aspect of the present invention, the ink-jet head having the above first feature is characterized in that each of the heating areas includes a protective film on the upper side of the heater film and the thermal conductivity of each protective film is made different from that of the others so as to produce difference in thermal efficiency.
In accordance with the sixth aspect of the present invention, the ink-jet head having the above first feature is characterized in that each of the heating areas includes a protective film on the upper side of the heater film and the thickness of each protective film is made different from that of the others so as to produce difference in thermal efficiency.
In accordance with the seventh aspect of the present invention, the ink-jet head having the above first feature is characterized in that each of the heating areas includes a protective film on the upper side of the heater film and the ratio of the thermal conductivity to the thickness of the protective film is made different from that of others so as to produce difference in thermal efficiency.
In accordance with the eighth aspect of the present invention, the ink-jet head having any one of the above first through seventh features is characterized in that the heating areas are arranged on a line joining between the nozzle and the ink supply port for supplying ink to the pressure chamber, so that the heating area closest to the nozzle has the highest thermal efficiency and the thermal efficiency varies continuously.
In accordance with the ninth aspect of the present invention, a control method of an ink-jet head, comprises the steps of:
using an ink-jet head for ejecting ink droplets from a nozzle by the pressure caused by bubbles, which comprises:
a pressure chamber communicating with the nozzle; and a plurality of heating areas disposed inside the pressure chamber for generating bubbles by heat generation, wherein heater films arranged in the heating areas are electrically connected in parallel and the surfaces of the heating areas facing the pressure chamber have different thermal efficiencies; and
controlling the applied energy to the heating areas in accordance with the density of the image to be recorded so as to vary the amount of ink droplets and perform recording of tones.
Adoption of the above first configuration makes it possible to select heating areas where bubbles should be generated by varying the applied energy level, and hence enables multilevel control of the ejected amount of ink droplets over a wide range of applied energy. As a result, it is possible to realize recording of multiple tones. Since the heater films contained in the heating areas are electrically connected in parallel, if any one of the interconnections connected to one of the heater films is disconnected, the ejection of ink will not be stopped by the disconnection only, thus making it possible to maintain reliable, high printing quality over a long period of time.
Adoption of the above second through seventh configurations makes it possible to easily make a difference in thermal efficiency between the heating areas and hence enables multilevel control of the ejected amount of ink droplets over a wide range of applied energy. As a result, it is possible to realize recording of multiple tones.
In the above eighth configuration, ink is preliminarily heated before the ink reaches the main heating area to a certain degree though it does not reach the temperature at which ink bubbles, through the other heating areas where they have lower thermal conductivities. As a result, the energy required for the ink to bubble in the heating area having a high thermal conductivity can be reduced compared to the case where the heating area having a high thermal conductivity is provided solo.
Adoption of the above ninth configuration enables multilevel control of the ejected amount of ink droplets over a wide range of applied energy. As a result, it is possible to realize recording of multiple tones. Since the heater films contained in the heating areas are electrically connected in parallel, if any one of the interconnections connected to one of the heater films is disconnected, the ejection of ink will not stop only by the disconnection, thus making it possible to maintain reliable, high printing quality over a long period of time.